Glow plug adn heater assembly therefor with an improved connection between a central electrode and a heater probe of the heater assembly

ABSTRACT

A glow plug has an annular metal shell and a heater assembly including a central electrode and a heater probe. The metal shell is configured to receive the heater assembly therein. The central electrode has an elongate body extending between a terminal end configured for electrical communication with a power source and an attachment end. The heater probe has an elongate body extending between a proximal end and a free distal end. Either the attachment end of the central electrode or the proximal end of the heater probe has a recessed pocket with a tapered inner surface converging axially extending axially therein. The other of the attachment end of the central electrode or the proximal end of the heater probe has a tapered outer surface configured for receipt in the recessed pocket.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates generally to glow plugs, and more particularly to glow plugs having a central electrode connected to a ceramic heater probe.

2. Related Art

A conventional ceramic glow plug may use a variety of mechanisms to connect an electrode to the heating part. If we consider coaxial connection, two types are in common use. A first type, which is shown in a fragmentary schematic cross-sectional view in FIG. 1, includes a central electrode 1 that is attached to a heater probe 2 via a connection joint 3. The central electrode 1 extends coaxially from the heater probe 2 and is generally configured for electrical connection to a power source. Typically, the connection joint 3 between the central electrode 1 and the heater probe 2 is established by a joint which is partially a butt joint, wherein respective planar end surfaces 4, 5 that face one another are brought into abutment with one another. The butt joint results in a purely tensile force being established across a relatively low surface area of the joint. In addition, an end portion of the heater probe is typically encapsulated by a cylindrical cup or housing 6 that increases the overall diameter of the assembly.

There is a requirement for a glow plug pressure sensor (GPPS) to apply tensile force to the central electrode 1, which in turn, exerts a force tending to separate the central electrode 1 from the heater probe 2, as illustrated schematically in FIG. 1B. In addition, there is typically a requirement that the central electrode 1 be coaxially aligned with the heater probe 2, and that the overall diameter of the assembly not be increased at the connection joint 3 between the central electrode 1 and the heater probe 2. As such, the attachment between the central electrode 1 and the heater probe 2 needs to withstand the applied tensile force and stresses generated thereby, while also not having an increased overall diameter in the region of connection between the central electrode and the heater probe. Accordingly, the manufacturing tolerances of the components 1, 2 need to be tight in order to provide precise surfaces for bonding to one another, as well as to provide an axially aligned assembly. The narrow tolerance limits required increase associated costs of manufacturing the heater assembly, and unfortunately, the aforementioned butt joint, even if carefully provided, is typically not able to withstand a significant tensile force, as illustrated in FIG. 7.

A second type of connection, illustrated in FIG. 1C, contains a recess in the end of the heating part 2′ into which an electrode 1′ is inserted and joined at a connection joint 3′, typically by brazing. This limits the maximum possible diameter of the electrode 1′ and provides a very small joint area, leading to poor performance under tensile load, as illustrated in FIG. 7: the joint 3′ may fail due to its small area or the small cross-section of the electrode 1′ may lead to its breakage.

SUMMARY OF THE INVENTION

A glow plug includes an annular metal shell, a central electrode and a heater probe. The metal shell has a central cavity configured to receive the central electrode and heater probe therein. The central electrode has an elongate body extending between a terminal end configured for electrical communication with a power source and an attachment end, and the heater probe has an elongate body extending between a proximal end and a free distal end. Either the attachment end of the central electrode or the proximal end of the heater probe has a recessed pocket with a tapered inner surface converging axially therein. The other of the attachment end of the central electrode or the proximal end of the heater probe has a tapered outer surface configured for receipt in the recessed pocket.

In accordance with another aspect of the invention, a heater assembly for a glow plug is provided. The heater assembly has a central electrode and a heater probe. The central electrode has an elongate body extending between a terminal end configured for electrical communication with a power source and an attachment end. The heater probe has an elongate body extending between a proximal end configured for attachment to the attachment end of the central electrode and a free distal end. Either the attachment end of the central electrode or the proximal end of the heater probe has a recessed pocket with a tapered inner surface converging axially therein. The other of the attachment end or the proximal end has a tapered outer surface configured for receipt in the recessed pocket.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects, features and advantages of the invention will become more readily appreciated when considered in connection with the following detailed description of presently preferred embodiments and best mode, appended claims and accompanying drawings, in which:

FIG. 1A is a partial schematic view of a central electrode and heater probe joined across a butt joint according to prior art;

FIG. 1B is a similar view to FIG. 1A showing the butt joint separating under a tensile force;

FIG. 1C is a partial schematic view of a central electrode and heater probe joined according to prior art;

FIG. 2 is a cross-sectional view of a glow plug having a heater assembly with a connection joint constructed according to one aspect of the invention;

FIG. 3 is an exploded fragmentary cross-sectional view of a central electrode and heater probe of the heater assembly of FIG. 2;

FIG. 4 is a cross-sectional view taken generally along the line 4-4 of FIG. 3;

FIG. 5 is a cross-sectional view taken generally along the line 5-5 of FIG. 3;

FIG. 6 is a graph illustrating the displacement of the central electrode relative to the heater probe versus applied tensile force;

FIG. 7 is a graph illustrating the tensile breaking strength and variation of tensile breaking strength of the prior art and the connection joint of FIG. 2; and

FIG. 8 is a view similar to FIG. 3 of a central electrode and heater probe of a heater assembly constructed in accordance with another aspect of the invention.

DETAILED DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS

Referring in more detail to the drawings, FIG. 2 illustrates a glow plug 10 having a heater assembly, identified generally at 12, constructed in accordance with one aspect of the invention. The heater assembly 12 has a central electrode 14 attached to a heater probe 16 via a tapered joint 18. The tapered joint 18 provides a bond between the central electrode 14 and the heater probe 16 having an increased surface area, thereby enhancing the strength of the bond between the central electrode 14 and the heater probe 16, which is particularly important in applications where a tensile force (F) is applied across the joint 18, as illustrated in FIGS. 1A and 1B of the prior art. In addition, the tapered joint 18 facilitates aligning the central electrode 14 and the heater probe 16 coaxially with one another along a central longitudinal axis 20 of the heater assembly 12. Accordingly, the tapered joint 18 acts to self-align the central electrode 14 the heater probe 16. These and other aspects of the heater assembly 12 allow the glow plug 10 to be economical in manufacture, while also providing the glow plug 10 with a long and useful life.

The glow plug 10 has an outer tubular metal shell 22 with an inner surface 24 providing a cavity 26 sized for receipt of the heater probe 16 at least partially therein. The cavity 26 has a reduced diameter portion 28 extending from one end 29 of the shell 22 to an enlarged diameter portion 30 at an opposite end 32 of the shell 22. The enlarged diameter portion 30 is configured for attachment to an upper tubular shell (not shown), wherein the upper tubular shell can be configured for attachment to an engine block.

The central electrode 14 has an elongate body 34 extending between a proximal or terminal end 36 configured for electrical communication with a power source and a distal or attachment end 38. The central electrode 14 can be constructed from any suitable conducting electrode material, such as metal. The body 34 has an outer surface 40 extending between the opposite ends 36, 38. A recessed pocket 42 extends into the attachment end 38. The pocket 42 is provided by an annular wall 44 having an electrically conductive, tapered inner surface 46 and a tapered outer surface 48 that is opposite and radially outward from the inner surface 46. The tapered inner surface 46 converges axially into the attachment end 38, and is illustrated here as extending to a generally flat base 50 to provide the pocket 42 with a frustroconical shape. The tapered inner surface 46 preferably extends along an interior or included angle 52 that is greater than an exterior angle 54 of the tapered outer surface 48. The tapered inner surface 46 can be provided having an angle between about 10-16 degrees, while the tapered outer surface 48 can be provided having an angle between about 10-14 degrees, depending on the angle of the inner surface 46, although other angles outside these ranges could be used. Accordingly, as shown in FIGS. 4 and 5, the annular wall 44 has a thickness 56 adjacent the base 50 that is greater than a thickness 58 adjacent the attachment end 38, wherein the thickness of the wall 44 can vary continuously from the attachment end 38 to the base 50. However, it should be recognized that the interior and exterior angles 49, 50 could be provided as substantially the same, thereby providing the wall 44 with a substantially constant wall thickness, if desired.

The heater probe 16 has an elongate ceramic body 60 extending between a proximal end 62 and a free distal end 64. The proximal end 62 has a tapered outer surface 66 that is configured to conform and mate with the tapered inner surface 46 of the central electrode 14. Accordingly, the tapered outer surface 66 preferably has an exterior angle 68 substantially the same as the tapered included angle 52 of the inner surface 46, and can be formed having a conical or frustroconical shape, depending on the shape of the recessed pocket 42. The tapered outer surface 66 can be fabricated conjointly with the body 60, for fabricated in a secondary machining operation, such as in a grinding process. The body 60 is shown here, by way of example and without limitation, as having a straight cylindrical portion 70 extending from the tapered outer surface 66 to the distal end 64, although other configurations can be used, if desired.

The heater probe 16 has a heating element represented generally at 72. Connection to the heating element 72 is provided by an electrically conductive region 74 on the tapered outer surface 66 and another electrically conductive region 82 on the outer surface of the cylindrical portion 70. An electrical circuit formed inside the heating probe 16 forms heating element 72, here represented as resistance 80. The conductive region 74 is configured to establish electrical communication with the tapered inner surface 46 of the central electrode 14, while the other conductive region 82 is configured to establish an electrical ground contact with the inner surface 24 of the metal shell 22. A pair of electrical leads 76, 78 establish electrical communication between the conductive regions 74, 82, while the main area of heating at resistance element 80 is located adjacent the distal end 64 and selected having a suitable resistance to generate the desired intensity of heat at the distal end 64. It should be recognized that the aforementioned components of the heating element 72 can be provided using any suitable metallic or ceramic materials, as desired.

In attaching the central electrode 14 to the heater probe 16, the tapered inner surface 46 and the tapered outer surface 66 provide a self-centering feature to bring the electrode 14 and the probe 16 into coaxial alignment with one another. Accordingly, in manufacture of the heater assembly 12, manufacturing tolerances can be relatively broad, since any slight differences in angles of the inner and outer surfaces 46, 66 will only result in a slight difference in overall length of the heater assembly 12, which is typically not a dimension of critical concern. In addition, upon attaching the central electrode 14 to the heater probe 16, the tapered joint 18 preferably extends about the abutting annular surfaces of the inner and outer tapered surfaces 46, 66, thereby providing an increased joint area over the prior art, while also placing any tensile forces (F) applied on the central electrode in both tension and mostly shear across the joint 18. As such, as illustrated in FIG. 6, the joint 18 is able to withstand a significant tensile force, as the displacement of the central electrode 14 relative to the heater probe 16 of the heater assembly 12 is greatly reduced over the prior art (illustrated prior art line is with reference to the joint shown in FIGS. 1A and 1B) by the increased strength of the joint 18. In addition, as shown in FIG. 7, the mean tensile breaking strength of the joint 18 exhibits an ability to withstand a dramatically increased tensile force (N) than the prior art joints 3, 3′ depicted in FIGS. 1A-1C. Further yet, the tapered joint 18 provides the heater assembly 12 with an outer diameter 84 that is minimized, and is shown here as being less than the outer diameter of the heater probe 16. In addition, with the annular wall 44 having a varying thickness, a generally constant cross-sectional area is provide across the annular wall 44 and the tapered proximal end 62 of the heater probe 16, thereby equalizing any stress in the electrode material during tensile loading.

In FIG. 8, another heater assembly 112 is illustrated in accordance with another aspect of the invention, wherein the same reference numerals offset by a factor of 100 are used to identify similar features as described above. The heater assembly 112 includes a central electrode 114 and a heater probe 116 attached in electrical communication with one another. The central electrode 114 has a recessed pocket 142 extending into an attachment end 138, however, rather than being conical or frustroconical, the recessed pocket has an inner surface 146 that provides the pocket 142 with a concave shape. Accordingly, the heater probe has a proximal end 162 that has a convex outer surface 166 configured for receipt in the pocket 142 of the central electrode 114. As such, in attaching the central electrode 114 and the heater probe 116 to one another, an increased degree of freedom is provided, given the central electrode 114 and the heater probe 116 can be pivoted relative to one another during assembly. This allows for reduced tolerance requirements in manufacturing, while still providing an increased surface area across a joint formed between the concave and convex surfaces 146, 166.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. 

1. A glow plug, comprising: an annular metal shell with a central cavity; a central electrode having an elongate body extending between a terminal end configured for electrical communication with a power source and an attachment end; a heater probe having an elongate body extending between a proximal end and a free distal end; and wherein one of said attachment end of said central electrode or said proximal end of said heater probe has a recessed pocket extending axially therein, said recessed pocket having a tapered inner surface converging axially into one of said attachment end or said proximal end, and the other of said attachment end of said central electrode or said proximal end of said heater probe has a tapered outer surface configured for receipt in said recessed pocket.
 2. The glow plug of claim 1 wherein said central electrode has said recessed pocket and said heater probe has said tapered outer surface.
 3. The glow plug of claim 2 wherein said tapered inner surface and said tapered outer surface mate with one another to axially align said central electrode with said heater probe.
 4. The glow plug of claim 2 wherein said tapered outer surface has an electrically conductive outer surface configured for electrical communication with said tapered inner surface of said central electrode.
 5. The glow plug of claim 4 wherein said heater probe has a substantially straight cylindrical outer surface extending from said tapered outer surface, said substantially straight cylindrical outer surface having an electrically conductive outer region configured for electrical communication with said shell.
 6. The glow plug of claim 5 wherein said heater probe has an electrical circuit extending from said electrically conductive outer surface to said electrically conductive outer region.
 7. The glow plug of claim 1 wherein said tapered inner surface of said recessed pocket is frustroconical in shape.
 8. The glow plug of claim 1 wherein said recessed pocket is formed by an annular wall having an outer surface radially outward of said tapered inner surface, said outer surface being tapered at a different angle than said tapered inner surface.
 9. The glow plug of claim 8 wherein said annular wall extends from one of said attachment end or said proximal end to a base of said recessed pocket, said annular wall having one thickness adjacent said base and another thickness adjacent one of said attachment end or said proximal end, said one thickness being greater than said another thickness.
 10. The glow plug of claim 9 wherein said thickness of said annular wall varies continuously extending from one of said attachment end or said proximal end to said base.
 11. A heater assembly for a glow plug, comprising: a central electrode having an elongate body extending between a terminal end configured for electrical communication with a power source and an attachment end; a heater probe having an elongate body extending between a proximal end configured for attachment to the attachment end of the central electrode and a free distal end; and wherein one of said attachment end of said central electrode or said proximal end of said heater probe has a recessed pocket extending axially therein, said recessed pocket having a tapered inner surface converging axially into one of said attachment end or said proximal end, and the other of said attachment end of said central electrode or said proximal end of said heater probe has a tapered outer surface configured for receipt in said recessed pocket.
 12. The heater assembly of claim 11 wherein said central electrode has said recessed pocket and said heater probe has said tapered outer surface.
 13. The heater assembly of claim 12 wherein said tapered inner surface and said tapered outer surface mate with one another to axially align said central electrode with said heater probe.
 14. The heater assembly of claim 12 wherein said tapered outer surface has a conductive outer surface configured for electrical communication with said tapered inner surface of said central electrode.
 15. The heater assembly of claim 14 wherein said heater probe has a substantially straight cylindrical outer surface extending from said tapered outer surface, said substantially straight cylindrical outer surface having an electrically conductive outer region.
 16. The heater assembly of claim 15 wherein said heater probe has an electrical lead extending from said electrically conductive outer surface to said electrically conductive outer region.
 17. The heater assembly of claim 11 wherein said recessed pocket is formed by an annular wall having an outer surface radially outward of said tapered inner surface, said outer surface being tapered at a different angle than said tapered inner surface.
 18. The heater assembly of claim 17 wherein said annular wall extends from one of said attachment end or said proximal end to a base of said recessed pocket, said annular wall having one thickness adjacent said base and another thickness adjacent one of said attachment end or said proximal end, said one thickness being greater than said another thickness.
 19. The heater assembly of claim 18 wherein said thickness of said annular wall varies continuously from one of said attachment end or said proximal end to said base. 